Nanomaterials and notably carbon nanotubes are presently causing a particular fascination because of their original and exacerbated properties as compared with conventional materials. Carbon nanotubes notably form a novel promising material for organic electronics.
First of all, let us recall that a carbon nanotube is defined as a concentric winding of one or more graphene layers (carbon hexagonal nets). The term “Single Wall NanoTube” (SWNT) is used when a single layer of graphene is used and “Multi Wall NanoTube” (MWNT) is used in the case of several graphene layers. Due to their unique structure and their dimensions characterized by a high length/diameter ratio, nanotubes have exceptional mechanical, electrical and thermal properties.
In particular, depending on the concentric winding of the graphene layer making them up, SWNTs may either be metal, i.e. conductive with an electric resistance as low as 1 to 2 kΩ for a diameter of 1 to 2 nm, or semiconducting with an gap gradually increasing as the diameter decreases (typically from 0.5 to 1.2 eV). Significant scientific literature has demonstrated the benefit of SWNTs, because of their high conductivity and their metal or semiconductor nature, in electronic applications in field effect transistors, in high frequency devices, in variable resistors, as pixels, as a chemical detector in a liquid or gas phase, etc.
However, these studies emphasize on the difficulties due to the fact that metal SWNTs (m-SWNTs) and semiconductor SWNTs (sc-SMWTs) are generally synthesized simultaneously in a ratio close to 1:2 and this regardless of the applied synthesis method for the SWNTs. Even if a few examples are found showing that it is possible to synthesize specific nanotubes with good selectivity, the authors deplore the fact that the window of associated experimental parameters is very narrow (Marquardt et al. 2008 [1]). Thus, applications mainly using the high conductance of m-SWNTs are hindered by the low and unstable conductance of sc-SWNTs present in the mixture. Also, applications using the semi-conductor gap of sc-SWNTs are hampered by short circuits produced by the m-SWNTs. A certain number of studies were focused on the separation or the enrichment of SWNTs according to the diameter or the chirality in order to obtain samples enriched in m-SWNTs or in sc-SWNTs.
The first and the most efficient up to now of these methods is a differentiation method based on density. In this technique, the SWNTs are dissolved in water by using one or more surfactants. The surfactants surround the SWNTs, the surfactant-SWNT association being selective depending on their metal or semiconductor type. Surfactant-SWNT complexes have different densities and may be separated by ultracentrifugation on a density gradient (Arnold et al., 2006 [2]). As ultracentrifugations are carried out on limited volumes, the purified amounts are inevitably limited but samples may be obtained with a good degree of purity.
Similarly, the difference in the association with surfactants may be used for obtaining a difference in the overall charge of the SWNTs. International application WO 2003/084869 [3] describes a separation method based on different protonation of the SWNTs surrounded by surfactants, in solution, according to their chirality (n,m). At a given pH, a voltage is applied in order to attract the charged nanotubes towards the electrode and a fraction is collected. The method is repeated at different pHs allowing collection of fractions enriched with different chiralities (n,m).
Also, the DNA may be used as a polymeric surfactant for solubilization of SWNTs, and oligomers with different sequence and length have been used for separating SWNTs. The SWNT-DNA addition products may then be separated by iron exchange chromatography, the first fractions being enriched with metal SWNTs of small diameter and the last fractions in semiconductor SWNTs, of a large diameter. Patent application US 2006/0223068 [4] relates to SWNT-DNA hybrids and more particularly to the solubilization of SWNTs by surrounding them with DNA. DNA surrounds the SWNTs in order to form a rather unstable soluble hybrid. International application WO 2004/048256 [5] proposes a method for separating m-SWNTs and sc-SWNTs based on their solubilization by using short single strand DNA molecules. Once they are solubilized according to this method, the small diameter SWNTs may be separated by using standard chromatography, electrophoresis, iron exchange chromatography or two-phase systems.
Complexation may be used in a simpler way and this by solubilizing a portion of the SWNTs in a poor solvent. Indeed, SWNTs are insoluble in water and in most of the organic solvents. Ligands selected for their selectivity towards the chirality or the diameter of the SWNTs such as porphyrin, which is a specific ligand of sc-SWNTs may be used for surrounding them and solubilizing them. International application WO 2006/013788 [6] describes a method for separating m-SWNTs and sc-SWNTs based on their different solubility in amines.
The m-SWNTs and sc-SWNTs may also be separated according to their electromagnetic properties. The difference in the polarizability of m-SWNTs and of sc-SWNTs has already been utilized for separating them. Indeed, the dielectrophoretic attractive force applied by a high-frequency AC electric field on m-SWNTs is greater than that of sc-SWNTs. International application WO 2005/030640 [7] describes a separation method based on the difference in polarizability and based on optical trapping for the separation. The SWNTs are trapped by a focused laser beam and may be displaced in a microfluidic system from one channel to the other by displacing the laser beam. The SWNTs of different chiralities are trapped at different wavelengths of the laser, so that the SWNTs may be displaced according to their chirality.
Mention may further be made of methods for separating m-SWNTs and sc-SWNTs based on their capability of giving electrons, itself related to their conductivity or to their difference in magnetic moment.
Other separation methods are based on the different chemical reactivity of metal SWNTs vs. Semiconductors SWNTS or large SWNTs vs. small SWMTs.
A first selective and simple treatment is oxidation of m-SWNTs of small diameter under aggressive conditions. International application WO 2005/041227 [8] proposes a method based on the application of a voltage on SWNTs deposited on a substrate in order to protect a type of SWNT, the non-protected type either being burned or chemically destroyed, for example by treatment with a strong acid. Selective oxidation of SWNTs has also been described with electrophilic compounds such as nitronium NO2+ ions, the attack of which functionalizes the large diameter sc-SWNTs and destroys the m-SWNTs of small diameter. Patent application US 2005/0255031 [9] is based on selective oxidation with nitronium ions. Less aggressive oxidation conditions cause preferential functionalization of m-SWNTs.
On the other hand, powerful reducing agents such as alkaline metals preferentially affect small diameter m-SWNTs. Reduction of SWNTs may be followed by alkylation with a view to separation.
Further, the inorganic functionalization of SWNTs generally involving the reduction of a salt at the surface of the SWNTs was also used for separating SWNTs. Patent application US 2007/0258880 [10] describes a method for separating m-SWNTs and sc-SWNTs based on photochemical reduction of a metal salt at the surface of the SWNTs in solution. The photochemical reaction is selectively induced on a SWNT type by irradiation of the solution at their absorption wavelength. SWNTs coated with metal may be separated from the solution under the action of a magnetic field.
The formation of covalent bonds at the surface of SWNTs usually has some selectivity of m-SWNT vs. sc-SWNT. 1,3 dipolar addition on the double bonds of SWNTs was recently used for selective functionalization of sc-SWNTs and their selective solubilization (Article of Menard-Moyon et al, 2006 [11]). On the contrary, addition of diazonium is selective for m-SWNTs. Thus, patent application JP 2007 031238 [12] describes a method for separating sc-SWNTs from m-SWNTs by derivatization by using a diazonium salt which preferentially derivatizes m-SWNTs. Metal nanoparticles are then coupled with aryl entities and the nanoparticles are removed. The solution of the remaining non-coupled SWNTs is enriched in sc-SWNTs and used for making thin film transistors.
Further, international application WO 2004/043857 [13] proposes a method for separating m-SWNTs and sc-SWNTs using a terminal bond to a polymer. The polymer promotes solubility in solution and the bond between the polymers and the SWNTs may be selectively broken by a heat treatment repeated at increasing temperatures. The SWNTs without a bound polymer are separated by filtration.
It should be emphasized that the separation of SWNTs according to their metal or semiconductor type seems to be easier to control than for small diameter SWNTs, i.e. for diameters of less than 1 nm (CoMoCat or HiPco SWNTs). Indeed, small diameter SWNTs show generally greater chemical reactivity, due to the strong curvature of the wound graphene sheet which induces a voltage on the C—C bonds. These samples also contain a more restricted number of different chiralities, giving the possibility of purifying a single chirality. Unfortunately, electronic devices are difficult to obtain from small diameter SWNTs. Further, devices based on sc-SWNTs can only be made at a large scale if the fraction based on sc-SWNTs are of sufficiently high purity in order to avoid short circuits of the m-SWNTs.
In the case of the present methods with partial separation, the separation step has to be repeated several times in order to attain a high level of purity, which considerably increases the purification cost. Therefore, novel improved and cost-effective separation methods are still required.